Homo Habilis.

Homo Habilis lived from about 2.4 to 1.5 million years ago, and is the earliest known species to show novel differences from the chimpanzee and australopithid skulls. The face is still primitive and projecting, but the jaw is pulled under the brain, with smaller molars (though still much larger than in modern humans), and the skull is thinner, with a distinctive rounded shape, vertical sides and a small forehead above the brows. The first humans have arrived on the scene. A male habilis may have stood at around 1.3 meters and weighed 37 kilos, and females 1.2 meters and 32 kilos. However, some forms of habilis were apparently smaller, and may have stood little more than a meter tall.

The two signature evolutionary trends in hominids are increasing brain size — in habilis, to an average of about 650cc — and a proportionate reduction in the size of the face. In habilis the brain shape is more humanlike: the bulge of Broca’s area, implicated in human language, is visible in at least one habilis brain cast. Another resemblance to modern humans is the reduced, less apelike or australopithid sexual dimorphism.

There’s evidence they ate meat, probably mostly scavenged or small creatures. They would still have eaten a lot of plant food, judging by the size of the molars. Homo Habilis itself was often on the menu for the large cats around at the time. Shaped stone tool use quite possibly goes back as far as the Australopithecus genus.

There’s been the suggestion that eating meat (nutirent packed) allowed us to eat the starchy roots (nutrient low), expanding our range of foods. Eating just the tubers and veg wouldn’t provide the nutrients required to build a brian. Vegetable sources of O3 oils aren’t that common, and are usually packed in with O6 oils to boot, making flesh consumption pretty crucial in the history of human brain development